ARM: davinci: constify gpio_led

gpio_led are not supposed to change at runtime.
struct gpio_led_platform_data working with const gpio_led
provided by <linux/leds.h>. So mark the non-const structs
as const.

Signed-off-by: Arvind Yadav <arvind.yadav.cs@gmail.com>
Signed-off-by: Sekhar Nori <nsekhar@ti.com>